To provide a power transmission mechanism capable of accumulating or releasing the rotation power of a shaft at an appropriate timing mainly with a mechanical structure only.

The power transmission mechanism 20 comprises a power spring 5 for temporarily accumulating the rotation power of the shaft S1, an accumulation side rotation power transmission mechanism 6 capable of transmitting the rotation power from the shaft S1 to the power spring 5, a release side rotation power transmission mechanism 7 capable of transmitting the rotation power from the power spring 5 to an output side sprocket 2, and a ball screw 8 for detecting the difference of the torque working on an input side sprocket 1 and the output side sprocket 2 respectively. When the accumulation side rotation power transmission mechanism 6 is operated based on the difference of the torque detected by the ball screw 8, the rotation power of the shaft S1 is accumulated in the power spring 5. When the release side rotation power transmission mechanism 7 is operated based on the difference of the torque detected by the ball spring 8, the rotation power accumulated in the power spring 5 is released to the output side sprocket 2.